– A ghost of Twins Past inflicted significant pain on his old team Tuesday night. But that’s nothing compared to the pain that struck Twins Future.

Danny Valencia, once a promising third baseman for the Twins, homered and doubled at O.co Coliseum, providing enough timely hitting to power the Oakland A’s to a 7-4 victory, Minnesota’s second straight loss.

But Miguel Sano, hustling down the first-base line to foil a double play and drive in a run, suffered a strained left hamstring and was immediately removed from the game. The Twins’ most dangerous hitter was placed on the disabled list after the game, and Max Kepler was summoned to California for Wednesday’s finale of the three-game series.

Watch the play on which Sano was injured

“It [hurts], because I want to help the team,” said Sano, whose 11 home runs and 27 RBIs lead the Twins. “It’s more frustrating because I was hitting the ball well.”

Sano’s absence, however long it turns out to be, represents a severe blow to a Twins offense that had begun to assert itself, however haltingly, over the past week. Minnesota’s home run binge, 11 in four games last week, included tape-measure shots from the 23-year-old cleanup hitter in all four games.

“It’s disappointing,” said Twins manager Paul Molitor. “It’s tough to have injuries, but you’ve got to deal with them along the way. They get bunched up once in awhile.”

Don’t the Twins know it. Sano’s departure marks the second straight day that a hamstring injury has hobbled a Twins starter. Center fielder Danny Santana, hurt while running the bases on Monday, was placed on the disabled list Tuesday. Byron Buxton, who doubled and struck out twice in four at-bats, replaced him.

Tyler Duffey, meanwhile, was battered by Oakland for much of the night, and he finished a disappointing May by allowing five runs, three of them driven home by Valencia. After the Twins took an early 1-0 lead on back-to-back doubles by Robbie Grossman and Byung Ho Park — the Korean slugger’s first extra-base hit since May 16 — Duffey surrendered an RBI triple to Billy Burns in the bottom of the inning, on a ball that Sano pulled up short of near the right-field line, then had to chase when it took a weird sideways bounce.

Watch Sano misplay Burns' triple

“It’s frustrating, because there were [four] times when we scored,” Molitor pointed out, “and they came right back and answered.”

Valencia rocketed a 3-1 fastball from Duffey, who allowed 16 runs in his final three May starts, in the third inning, a two-run blast that was his first home run ever against the team that drafted him a decade ago. And in the fifth, Valencia broke a 3-3 tie by doubling to deep center field, driving home Stephen Vogt from first base.

“It felt like I was behind everyone 2-0,” said Duffey, who fell to 2-4. “I was fighting my fastball command all night. When I went out there in the first inning, knew it was going to be a rough one.”

The loss meant the Twins ended May having trailed at some point in all but one game, same as they did in April. Minnesota has trailed in 49 of its 51 games this year.

Sano left on an RBI note, at least, a hustle play when he hit a one-out chopper to Valencia with the bases loaded in the third inning. The A’s tried to turn an inning-ending double play, but Sano stretched to reach the bag, narrowly beating the relay throw and allowing the run to score — but at a serious cost.

“When he gets going, he moves pretty well for a big man,” Molitor said. “He runs hard, he’s not one of the guys you worry about. That one, you smell an RBI, and he did a good job getting down there.”

Kepler, 2-for-12 in a brief stint with the Twins in April, was batting .286 with 10 extra-base hits, six of them triples, and 18 RBIs at Class AAA Rochester.

 Etc.

• Mauer was named AL Player of the Week on Monday, after collecting 11 hits in six games, four of them home runs, last week. It’s the third such honor of Mauer’s career, but his first since 2009. “I didn’t really realize I was having the week I was,” said Mauer, who receives a watch from MLB in recognition of the award.

• Alex Meyer, who hasn’t pitched since May 3 against the Astros, will have a cortisone injection into his pitching shoulder in hopes of relieving the pain that has nagged him for a month. An MRI found only inflammation and no structural damage, the Twins said. Meyer “feels great when he throws in the bullpen,” assistant general manager Rob Antony said, “but the next day, it’s sore again. … We’re hoping the cortisone quiets that down.”

• Closer Glen Perkins, feeling stronger as his throwing program advances, will pitch a bullpen session Saturday.

• Santana said Monday his left hamstring strain is in the same leg as the injury that sidelined him in April. But Twins trainers checked their records and reminded Santana that he was mistaken — the previous injury was to his right hamstring.